1. Field of the Invention
The present invention relates to an apparatus and method for processing biological information.
2. Description of the Related Art
Research on optical imaging apparatuses for irradiating biological tissue with light emitted from a light source such as a laser, and imaging biological information obtained on the basis of incident light is being advanced actively in the medical field.
One of optical imaging techniques is photoacoustic tomography (PAT) called photoacoustic imaging. The photoacoustic imaging is a method of obtaining a distribution of optical characteristic values in biological tissue using the properties of an ultrasonic wave that is scattered in vivo less than light (refer to U.S. Pat. No. 5,840,023 and M, Xu, L. V. Wang, “photoacoustic imaging in biomedicine”, Review of scientific instruments, 77, 041101, 2006 (hereinafter, referred to as “Non-patent Document 1”).
According to the method, biological tissue is irradiated with pulsed light generated from a light source to detect an acoustic wave generated from biological tissue which has absorbed the energy of the pulsed light propagated and diffused inside the biological tissue. Specifically, using the difference between the absorptance of optical energy of a region of interest, such as a tumor, and that of another tissue, a transducer receives an elastic wave generated from the region of interest when the region momentarily expands by absorbing the energy of applied light. Detected signals are analyzed, thus obtaining a distribution of optical characteristic values in the biological tissue, especially, a distribution of optical energy absorption densities (hereinafter, referred to as “optical energy absorption density distribution”).
One of such optical imaging techniques other than the PAT is diffuse optical tomography (DOT) called diffuse optical imaging. The diffuse optical imaging is a technique of irradiating biological tissue with light emitted from a light source, detecting weak light propagated and diffused in the biological tissue through a high-sensitive photodetector, and imaging a distribution of optical characteristic values in the biological tissue on the basis of detected signals.
Furthermore, one of imaging techniques using light and an ultrasonic wave is acousto-optical tomography (AOT). The AOT is a technique of irradiating biological tissue with light, applying a focused ultrasonic wave to a local region in the biological tissue, and detecting modulated light through a photodetector using such an effect (acousto-optic effect) that the light is modulated by the ultrasonic wave (refer to U.S. Pat. No. 6,957,096). It is known that the AOT and the PAT yield higher resolution than the DOT because these techniques each detect a signal of a local region where light and an ultrasonic wave interact with each other.
According to the PAT, an acoustic wave absorbed and generated in a local region of interest is measured, thus obtaining local optical absorption information. A pressure p of the acoustic wave generated in the region of interest is expressed using a distance z between the region and a light irradiation point by the following expression:P(z)=Γμa(z)Φ(z)  (1)where Γ denotes a Gruneisen coefficient (thermal-to-acoustic conversion efficiency), μa(z) denotes an absorption coefficient at a position in the distance z, and Φ(z) denotes the intensity of light (hereinafter, referred to as “light intensity”) at the position in the distance z. The Gruneisen coefficient Γ, serving as an elastic characteristic value, is obtained by dividing the product of an isobaric volume expansion coefficient β and a squared sound speed c by a specific heat Cp.
It is known that Γ takes a substantially constant value so long as a biological tissue is determined. Accordingly, a change in sound pressure P, indicating the magnitude of the acoustic wave, is measured in a time-resolved manner, thereby obtaining the product of μa and Φ, namely, an optical energy absorption density distribution H (refer to Non-patent Document 1).
To accurately image a distribution of absorption coefficients μa(z) in biological tissue on the basis of the sound pressure P as a measured value, the light intensity Φ(z) at the position z of the region of interest has to be estimated accurately, as will be understood from Expression (1).
According to the technique disclosed in Non-patent Document 1, as an approach to estimation of the light intensity Φ(z), an average light attenuation coefficient Φeff(γ) in biological tissue is used and the Lambert-Beer Law and the diffusion theory are used to obtain the light intensity Φ(z). Using the light intensity Φ(z), the absorption coefficient μa(z) is obtained based on the sound pressure P(z). The attenuation coefficient μeff(γ) is expressed by the following expression.μeff=√{square root over (3μa(μ′s+μa))}  (2)
In Expression (2), μ's denotes an equivalent scattering coefficient. When a specimen is optically homogeneous, Expression (2) can be used. If the specimen is heterogeneous optically, however, it is difficult to accurately estimate the light intensity Φ(z). For example, the specimen is irradiated with light and diffused light radiated from the specimen is measured, so that the attenuation coefficient μeff(γ) can be estimated. The attenuation coefficient μeff(γ) is seriously affected by the optical characteristics of part in the vicinity of the surface of the specimen. In addition, the intensity of light which has reached a region of interest relatively deeper than the surface is affected by the optical characteristics of heterogeneous tissues between the surface and the region. Unfortunately, the light intensity Φ(z) of the region of interest is extremely shifted from the light intensity estimated using the attenuation coefficient μeff(γ). If the light intensity Φ(z) is not estimated accurately, the absorption coefficient μa(z) of the region of interest is not obtained with high accuracy.